The present invention relates to a spark plug for use in an internal combustion engine, and more particularly to a spark plug which generates a spark discharge including a surface discharge (creeping discharge) that occurs along a top end surface of a ceramic insulator, in a spark discharge gap formed between a top end portion of a ground electrode and an outer circumferential surface of a top end portion of a center electrode.
In recent years, there have been proposed and developed various spark plugs forming a spark discharge gap between a ground electrode and a center electrode and generating a surface discharge along a top end surface of a ceramic insulator. As one such spark plug, for instance, an intermittent discharge type spark plug is known, and is disclosed in Japanese Patent Provisional Publication No. 2005-203119 (hereinafter is referred to as “JP2005-203119”. In the intermittent discharge type spark plug, under a normal condition, an aerial discharge, i.e. sparks through the air between one end portion of the ground electrode and a top end portion of the center electrode occur, whereas under a so-called smoldering condition in which the surface of the ceramic insulator is fouled (stained) with carbon, the spark discharge occurs in a path where the surface discharge along the top end surface of the ceramic insulator appears. Upon the occurrence of the spark discharge, the carbon adhering or deposited on the top end surface of the ceramic insulator is burned off and cleaning of the spark plug is performed, the aerial discharge then occurs again between the ground electrode and the center electrode. With regard to the ceramic insulator used in such spark plug, it is generally formed as follows. After press-molding an insulative ceramic powder (e.g. alumina) in an elastic or rubber mold together with a pin that is inserted for forming an axial hole, a compact is subjected to the cutting and the grinding processes so as to be shaped into an outside shape of the ceramic insulator. Subsequently, the pin is pulled out, and the compact is sintered then a glost firing process is carried out, the ceramic insulator is finally completed.